A packer is a device that is used in a well to form an annular seal between an inner tubular member and a surrounding outer tubular member (a casing string or a liner, as just a few examples) or borehole wall. As examples, the inner tubular member may be a tubular string (a test string, production string, work string, etc.) or may be part of a downhole tool (a formation isolation valve, bridge plug, etc.).
One type of conventional packer has a seal element that is formed from a set of elastomer seal rings. The rings are sized to pass through the well when the packer is being run downhole into position. When the packer is in the appropriate downhole position and is to be set, gages of the packer compress the rings to cause the rings to radially expand to form the annular seal.
A weight-set packer uses the weight of the string and possibly the weight of additional collars to compress the packer's seal rings. In this regard, when the packer is to be set, the string may be mechanically manipulated from the surface of the well to initiate the release of the weight on the rings.
A hydraulically-set packer uses fluid pressure to compress the seal rings. The fluid pressure may be, as examples, pressure that is communicated downhole through a tubing string; annulus pressure; or pressure that is communicated downhole through a control line.
Other types of packers may include seal elements that are set without using compression. For example, a packer may have an inflatable bladder that is radially expanded to form an annular seal using fluid that is communicated into the interior space of the bladder through a control line. As another example, a swellable packer has a swellable material that swells in the presence of a well fluid or other triggering agent to form an annular seal.